Metal oxide nanomaterials and their application in solar\ud photoelectrolysis of water

Doctoral thesis English OPEN
Kler, Rantej Singh (2014)
  • Subject: QD0415 | QD0241

Solar generated hydrogen as an energy source is green, sustainable, with a high\ud energy density. One day the majority of current fossil fuel based technology could\ud be replaced with hydrogen technology reducing CO2 emission drastically. The goal\ud in this research is to explore hybrid metal oxide photocatalysts in the pursuit of\ud achieving highly efficient photoanodes for use in photoelectrochemical cells (PEC).\ud Achieving high efficiencies of hydrogen production in photoelectrochemical cells is\ud the key challenge for the commercialisation of PEC technology as a viable, sustainable,\ud hydrogen source; limited only by the lifetime of the sun and the resources of\ud the metal oxide materials.\ud In this research TiO2, Fe-Ti-O, ZnO, and Zn2TiO4 are the photocatalysts explored.\ud Alloys of Ti-Fe-O showed improvement over TiO2, whilst a hybrid heterostructure\ud of ZnO/Zn2TiO4/TiO2 enhanced photocurrent densities significantly. A\ud barrier layer in the photoanode achieved localised exciton separation and reduction\ud of recombination rates by inhibiting back flow of electrons after injection into the\ud TiO2 layer.\ud Nanotubes are created by the simple electrochemical process of anodisation. The\ud nanotube composition depends on the anode material. To control the composition ofthe anode, iron and titanium are co-deposited onto a substrate using electron beam\ud evaporation. The introduction of iron into titania nanotubes engineered the band\ud gap, lowering the band gap energy to that of iron oxide whilst the positions of the\ud conduction and valence bands with respect to the oxidation and reduction potentials\ud of water remained favourable. Fe-Ti-O nanotubes showed remarkable photocurrent\ud density improvement compared to TiO2 nanotubes.\ud ZnO nanostructures deposited by vapour transport mechanisms showed variability\ud in the morphology of the structures, as governed by the growth dynamics.\ud Herein, it is shown that an electronically favourable situation arises by the formation\ud of a ZnO-Zn2TiO4-TiO2 heterostructure and a high photocatalytic activity is\ud reported. The structure is composed of a large surface area ZnO nanorod photoabsorber\ud formed on a Ti foil which forms a Zn2TiO4 barrier layer between ZnO and\ud TiO2. The Zn2TiO4 layer inhibits electron transport toward the surface of the photoanode\ud whilst encouraging charge transport to the hydrogenation electrode. The\ud heterostructure interfacial surface area is extended through the utilisation of TiO2\ud nanotubes, which demonstrated a 20.22 % photoelectrochemical efficiency under UV\ud illumination.\ud Surface modification of ZnO nanorods with aerosol assisted chemical vapour\ud deposited TiO2 nanoparticles enhanced photocurrent densities of the ZnO rods,\ud improving charge separation of excitons created within the TiO2 nanoparticles.\ud ZnO nanotubes formed via a novel route using chemical bath deposition of ZnO\ud is investigated, an annulus ZnO seed layer facilitated the site specific growth of ZnO\ud nanotubes whilst a uniform seed layer formed ZnO nanorods.
  • References (174)
    174 references, page 1 of 18

    1 Introduction 1 1.1 Current Non-Solar Renewable Energy Technologies . . . . . . . . . . 2 1.1.1 Biomass Energy . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1.2 Wind Turbine Technologies . . . . . . . . . . . . . . . . . . . 3 1.1.3 Hydroelectricity . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2 Solar Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2.1 Semiconductor Fundamentals . . . . . . . . . . . . . . . . . . 7 1.2.2 Solid-State Photovoltaics (PV) . . . . . . . . . . . . . . . . . 9 1.2.3 Photoelectrochemical Solar Cells (PEC) . . . . . . . . . . . . 12 Dye Sensitised Solar Cells (DSSC's) . . . . . . . . . . . . . . . 13 Photocatalytic Water Electrolysis . . . . . . . . . . . . . . . . 14 1.3 Utilising Nanostructured Metal Oxides . . . . . . . . . . . . . . . . . 17 Photoanode Fundamentals . . . . . . . . . . . . . . . . . . . . 17 Advantages of Nanomaterial Photoanodes . . . . . . . . . . . 20 1.4 Hydrogen, Fuel of the Future? . . . . . . . . . . . . . . . . . . . . . . 23 Current Methods (CO2 Emissive) of Hydrogen Production . . 23 CO2 Neutral Methods of Hydrogen Production . . . . . . . . . 24 Hydrogen Storage . . . . . . . . . . . . . . . . . . . . . . . . . 26 Fuel Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 1.5 Thesis Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

    2 Analytical Methods 32 2.1 Scanning Electron Microscope (SEM) . . . . . . . . . . . . . . . . . . 32 2.1.1 Electron Source . . . . . . . . . . . . . . . . . . . . . . . . . . 34 2.1.2 Morphology Imaging . . . . . . . . . . . . . . . . . . . . . . . 35 Evehard-Thornley Detector . . . . . . . . . . . . . . . . . . . 36 2.1.3 Compositional Analysis - EDX . . . . . . . . . . . . . . . . . . 37 2.2 X-ray Diffraction (XRD) . . . . . . . . . . . . . . . . . . . . . . . . . 38 2.3 Photo-Electrochemistry Overview . . . . . . . . . . . . . . . . . . . . 43 2.3.1 Experimental setup of a solar water splitting cell . . . . . . . 45 2.3.2 Light Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Xenon Lamp . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Xenon Lamp, with a UV Transmitting Filter Installed . . . . 48 Cold Cathode Fluorescent Lamp light source . . . . . . . . . 49 2.3.3 Efficiency Analysis of the Photoanodes . . . . . . . . . . . . . 49

    3 Synthetic Methods 52 3.1 Electron Beam Evaporation . . . . . . . . . . . . . . . . . . . . . . . 52 3.1.1 Maintaining a High Vacuum Environment . . . . . . . . . . . 55 Rough Rotary Pump . . . . . . . . . . . . . . . . . . . . . . . 56 Turbo Molecular Pump . . . . . . . . . . . . . . . . . . . . . . 57 Titanium Sublimation Pump . . . . . . . . . . . . . . . . . . . 57 Ion Pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 3.1.2 Deposition Conditions and Rates . . . . . . . . . . . . . . . . 59 3.2 Anodisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 3.3 Vapour Transport Synthesis . . . . . . . . . . . . . . . . . . . . . . . 64 3.3.1 Vapour Liquid Solid Growth & Vapour Solid Growth . . . . . 66 3.3.2 AACVD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 3.4 Chemical Bath Deposition (ZnO) . . . . . . . . . . . . . . . . . . . . 71

    4 Fe-Ti-O Nanotube Composite Formation via Anodisation. 73 4.1 TiO2 Nanotube Formation . . . . . . . . . . . . . . . . . . . . . . . . 74 4.2 Fe2O3 Nanotube Formation . . . . . . . . . . . . . . . . . . . . . . . 75 4.2.1 Ti Thin Film PVD on to a FTO Substrate . . . . . . . . . . . 78 4.3 Co-Evaporated Fe/Ti Thin Film, and the Formation of Nanotubes . . 80 4.4 TiO2 and Fe2O3 Nanotube Formation on Complementary Substrates . 83 4.4.1 Titanium and Iron stacked Thin Film on Conductive Glass . . 85 4.5 Photoelectrochemical Analysis . . . . . . . . . . . . . . . . . . . . . . 88 4.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

    5 Zinc Oxide Nanostructures formed by Vapour Transport Processing 95 5.1 Vapour Transport Nanostructure Synthesis . . . . . . . . . . . . . . . 96 5.1.1 Carbothermal ZnO decomposition . . . . . . . . . . . . . . . . 97 5.1.2 Direct Zn Powder Evaporation . . . . . . . . . . . . . . . . . . 103 5.1.3 Morphology Control . . . . . . . . . . . . . . . . . . . . . . . 106 5.2 XRD Analysis of the Crystal Structure of ZnO Nanorods Formed on a Ti Foil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 5.3 Photoelectrochemical Tests . . . . . . . . . . . . . . . . . . . . . . . . 120 5.4 Thickness Controlled Rods and Photocurrents . . . . . . . . . . . . . 125 5.5 Growth of ZnO Structures in a KOH Rich Environment . . . . . . . . 128 5.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

    6 Zinc Oxide Nanotubes and Rods on Titanium Nanotubes 135 6.1 Material Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 6.1.1 Titanium Dioxide Nanotubes . . . . . . . . . . . . . . . . . . 137 6.1.2 Zinc Oxide Nanotubular Growth . . . . . . . . . . . . . . . . 138 Initial Zinc Oxide Growth inside Tubes . . . . . . . . . . . . . 138 6.1.3 Growth of a Zinc Oxide Nanotubular Structure on a Titanium Dioxide Nanotube Framework . . . . . . . . . . . . . . . . . . 140 6.2 XRD analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 6.3 Photo-electrochemical Applications; the Splitting of Water . . . . . . 147 6.3.1 Experimental Analysis . . . . . . . . . . . . . . . . . . . . . . 147 6.3.2 Photoelectrochemical Performance of Photoanodes under different Illumination Sources . . . . . . . . . . . . . . . . . . . . 147 Photoanode Performance under Xenon light source Illumination147 Photoanode Performance under Xenon Light Illumination with a UV Transmission Filter Fitted . . . . . . . . . . . 149 Photoanode Performance under Cold Cathode Fluorescent Light (CCFL) Illumination . . . . . . . . . . . . . . . . . . 151 Comparison of Various Photoanodes . . . . . . . . . . . . . . 152 6.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

    [47] Alexander Volta and Joseph Banks. I. on the electricity excited by the mere contact of conducting substances of different kinds. The Philosophical Magazine: Comprehending the Various Branches of Science, the Liberal and Fine Arts, Agriculture, Manufactures, and Commerce, 7(28):289-311, 1800.

    [48] AE Becquerel. M´emoire sur les effets ´electriques produits sous l'influence des rayons solaires. Comptes Rendus, 9(567):1839, 1839.

    [50] Akira Fujishima. Electrochemical photolysis of water at a semiconductor electrode. nature, 238:37-38, 1972.

    [51] Lionel Vayssieres. On solar hydrogen and nanotechnology. John Wiley & Sons, 2010.

  • Related Research Results (1)
  • Metrics
    No metrics available
Share - Bookmark